Cryptology ePrint Archive: Report 2002/152

A note on Weak Keys of PES, IDEA and some Extended Variants

Jorge Nakahara Jr and Bart Preneel and Joos Vandewalle

Abstract: This paper presents an analysis of the PES cipher in a similar setting as
done by Daemen et al. at Crypto'93 for IDEA. The following results were
obtained for 8.5 round PES: a linear weak-key class of size
$2^{48}$; two distinct differential weak-key classes of size $2^{41}$; two
differential-linear weak-key classes of size $2^{62}$. For 17-round PES
(double-PES): a linear weak-key class of size $2^7$, and a differential
weak-key class of size $2^7$ were found. Daemen suggested a modified key
schedule for IDEA in order to avoid weak keys. We found a differential
weak-key class of size $2^{83}$ for 2.5-round IDEA under his redesigned
key schedule, and differential-linear relations for 3.5-round IDEA.